1. Field of the Invention
The present invention relates to an electric double layer capacitor, and more specifically, relates to an electric double layer capacitor having a pair of polarizing electrodes sandwiching therebetween a separator. The present invention also relates to a method for fabrication thereof.
2. Description of the Related Art
An electric double layer capacitor (EDLC) is known as an electric charge storage device which stores electric charge in an electric double layer formed at the interface between a polarizing electrode and an electrolyte. The structure of a basic cell for the EDLC is shown in FIG. 1. The EDLC includes a stacked cell structure including at least one basic cell (single basic cell in this example) sandwiched between a pair of thrust plates 19 by using bolts and nuts 20 and 21. The basic cell includes a pair of polarizing electrodes 11, a separator 12 sandwiched therebetween, a pair of current collectors sandwiching therebetween the polarizing electrodes 11 and the separator 12.
The polarizing electrode 11 is required to be stable in the presence of the electrolyte, and have excellent electric conductivity and a large surface area. Thus, activated carbon powder or activated carbon fiber is used as the polarizing electrode 11. The polarizing electrode 11 is also obtained by molding activated carbon with a binder such as polytetrafluoroethylene, as described in Japanese Patent Laid-Open Publication Hei 6-196364, or a solid-state activated carbon in which activated carbon is bonded with polyacene and carbon, as described in Japanese Patent Laid-Open Publications Hei 7-99141 and Sho 63-226019.
The electrolyte is largely categorized into two types including an aqueous solution type and an organic solvent type. As the aqueous solution type electrolyte, sulfuric acid or potassium hydroxide is mainly used, and as the organic solvent type electrolyte, quaternary ammonium salt or the like is mainly used. As a separator 12, porous films having an electrical insulating property and high ionic permeability are used, which include, for example, nonwoven fabrics such as glass fiber or polypropylene fiber, and polyolefine porous films. As the current collector 13, rubber or elastomer imparted with electric conductivity by carbon powder or the like is used in the case of electrolyte of the aqueous solution type, whereas a metallic film is used in the case of electrolyte of the organic solvent type.
A gasket 14 has a function of maintaining the shape of the basic cell and preventing the electrolyte from leaking, as well as preventing a short-circuit failure due to contact of top and bottom collectors 13. On the outside of the collector 13, there are provided terminal boards or lead terminals 15 electrically connected to the current collectors 13. In order to reduce the internal resistance of the basic cell, thrusting pressure is applied by the insulating thrust plates 19 from outside of the upper and lower terminal boards 15, and four corners of the thrust plate 19 are secured by bolts 20 and nuts 21.
The withstand voltage of the basic cell shown in FIG. 3 is determined depending on the electrolyte. When the aqueous solution type electrolyte is used, the withstand voltage is 1.0 V. When the organic solvent type electrolyte is used, it is about 2.0 to 3.0 V depending on the electrolyte to be used. In the cell structure of the EDLC, a plurality of basic cells are stacked one on another depending on the necessary withstand voltage.
The EDLCs have been used for applications of relatively small current, such as back up of semiconductor memory devices. On the other hand, recently, development for an application requiring a large current, such as energy regeneration in vehicles, no-service interruption power source in electronic equipment or the like, has been desired. In order to obtain the large current, it is desired to reduce the thicknesses of the electrodes 11 and the current collectors 13 and to reduce the equivalent series resistance (hereinafter referred to as xe2x80x9cESRxe2x80x9d) of the EDLC. Moreover, since the electronic equipment have been made small, the EDLC for use in such electronic equipment is also desired to have a lower thickness.
However, with the conventional EDLC shown in FIG. 1, there is a problem in that the contact resistance between the current collector 13 and the polarizing electrode 11 is large. There is also another problem in that even if this contact resistance is reduced by applying a thrust pressure from both the sides of the cell structure for fixing, the contact resistance eventually increases with the reduction of the applied pressure, thereby increasing the ESR. Moreover, with the conventional EDLC, there is another problem in that if it is used for a long time in a condition exceeding the working temperature range and the voltage range, peeling-off occurs between the current collector 13 and the polarizing electrode 11 and between the polarizing electrode and the separator 12 due to the gas generated inside the capacitor, and as a result, the ESR increases.
In order to solve those problems as recited above, it is considered to bond the polarizing electrode and the current collector, and to bond the polarizing electrode and the separator. As a method of bonding the polarizing electrode and the current collector, there can be mentioned a method of bonding these with an adhesive strength that the current collector originally has, or a method of bonding these with a conductive adhesive, as shown in Japanese Patent Laid-Open Publications Hei 05-082396 and Hei 11-154360.
On the other hand, with regard to the method of bonding the polarizing electrode and the separator, since it is difficult to impart an adhesive strength to either of the polarizing electrode or the separator, there is no example reported to date.
It is therefore an object of the present invention to suppress a change in the ESR of the EDLC due to a change in the applied pressure from both the ends of the cell structure to realize a long-term reliability.
The present invention provides a method for fabricating an electric double layer capacitor comprising the consecutive steps of: sandwiching a separator between a pair of polarizing electrodes; bonding the separator to the pair of polarizing electrodes; forming a pair of current collectors in electric contact with respective the polarizing electrodes to obtain a basic cell; forming a cell structure including at least one the basic cell and a pair of lead terminals in electric contact with the current collectors disposed outermost sides of the cell structure.
In accordance with the method of the present invention, the EDLC fabricated by the method has an excellent ESR property and a long lifetime for operation. In particular, the bonded structure of the polarizing electrodes and the separator affords prevention of the change of the ESR and peeling-off between the polarizing electrodes and the separator. The bonding structure may be preferably obtained by a thermal fusion between the polarizing electrodes and the separator by using a specific material for the separator.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.